1. Field of the Invention
The present invention relates to a phase controller for an ac motor installed in an electrically-powered tool.
2. Description of Related Art
A conventional phase controller for an ac motor installed in an electrically-powered tool is shown in FIG. 4. The phase controller 1 is in circuit with an ac motor M and an ac power supply AC, and it comprises a power switch SW1, a high-rotation switch SW2, a thyristor SCR, a bidirectional switching device DIAC, a variable resistor R.sub.1 operatively connected to the operating lever or trigger (not shown) of the electrically-powered tool, a diode D and a capacitor C.
The ac motor M is connected on one side to one side of the ac power supply AC, and the ac motor M is connected on the other side to the other side of the ac power supply AC through the series-connection of the power switch SW1 and the high-rotation switch SW2.
The thyristor SCR and a series-connection of the variable resistor R.sub.1 and the capacitor C are parallel-connected with the high-rotation switch SW2.
The bidirectional switching device DIAC is connected between the gate electrode of the thyristor SCR and the joint between the variable resistor R.sub.1 and the capacitor C, and the capacitor C is parallel-connected with the diode D.
The so constructed phase controller 1 is installed in an electrically-powered tool so that the power switch SW1 and high-rotation switch SW2 of the phase controller 1 may be made to turn on or off, depending on how much the operating lever or trigger is depressed. Also, the resistance of the variable resistor R.sub.1 varies with the degree of depression of the operating lever, thereby permitting the rotating speed of the ac motor M to be controlled in terms of the variable resistance. The operation of the phase controller is described for ac power supplies of 50 and 60 Hz, respectively.
Referring to FIG. 5A, when the ac power supply of 50 Hz is used, the operating lever is pressed to permit the power switch SW1 to turn on, thus starting the charging of the capacitor C through the variable resistor R.sub.1. When the charging voltage across the capacitor C rises above the break-over voltage DV of the bidirectional switching device DIAC (approximately 32 to 36 volts), the thyristor SCR is put in conductive condition from its anode-to-cathode electrode, thus supplying the ac motor M with electric power. The rate at which the charging voltage rises up to the break-over voltage DV of the bidirectional switching device DIAC is determined by the resistance of the variable resistor R.sub.1. The more the operating lever is depressed, the sooner the charging voltage rises up to the break-over voltage DV.
As the operating lever is depressed, the resistance of the variable resistor R.sub.1 decreases, and accordingly the charging rate of the capacitor C increases, thereby causing the angle of ignition of the thyristor SCR to lead for earlier position, and accordingly permitting the motor M to run at an increasing speed. On the contrary, as the operating lever is released, the resistance of the variable resistor R.sub.1 increases, and accordingly the charging rate of the capacitor C decreases, thereby causing the ignition angle of the thyristor SCR to lag for later position, and accordingly permitting the motor M to run at a decreased speed.
Referring to FIG. 5(B), the phase controller is connected to an ac power supply of 60 Hz, permitting the capacitor C to be charged with electricity at the rate of time constant given by the variable resistance R.sub.1 times the capacitance C, and accordingly the quantity of electric power fed to the ac motor M is controlled. More specifically, the capacitor C is charged repeatedly with electricity until the bidirectional switch DIAC turns on, triggering the thyristor SCR from its blocking to its conducting state in each positive half of the applied ac voltage for putting the ac motor M in running condition. It should be noted that the ignition angle lags much behind, compared with FIG. 5(A).
FIG. 6 is a graphic representation of the motor running rate (ordinate) versus the lever stroke or depression (abscissa). As for the initial stroke required for starting the motor M: the stroke difference W1 between the stroke for 50 Hz and the stroke for 60 Hz is 0.5 mm for 80 volts; the stroke difference W2 between the stroke for 50 Hz and the stroke for 60 Hz is 2 mm for 100 volts; and the stroke difference W3 is 4 mm for 110 volts (determined by extrapolation as indicated by broken lines). This initial stroke difference for 110 volts is fairly large.
Also, as seen from the graphic representation, the stroke difference is significantly large at the outset for respective voltages, but as the running rate rises, the stroke difference will decrease, converging at the coordinates of the stroke of 6.5 mm and the running rate of 80%, almost maximum speed. The more the initial stroke is large, the steeper the running rate rises toward the maximum speed.
As may be understood from the above, an electrically powered tool equipped with the conventional phase controller cannot be used at same running speed without depressing and moving its lever over a different distance for 50 or 60 Hz. This stroke difference is caused by the difference between the period for 50 Hz and 60 Hz (the period for 60 Hz being shorter than that for 50 Hz). Referring to FIGS. 5(A) and 5(B) again, the length of time T required for charging the capacitor C with electricity within the positive half of the waveform is almost equal for 50 and 60 Hz, and accordingly the remaining length of time (hatching area) allotted to the driving of the ac motor M for 50 Hz is longer than that for 60 Hz, thus permitting the ac motor M to be powered for a longer time for 50 Hz than for 60 Hz. This is the cause for the stroke difference for same running speed between ac power supplies of 50 and 60 Hz.
Also, disadvantageously if the voltage across the power supply fluctuates more or less, say .+-.10%, the rate at which the capacitor can be charged with electricity varies significantly, and the ignition angle of the thyristor SCR varies accordingly, and hence, the running speed of the ac motor M varies accordingly. Assume that the controlling parameters of an electrically powered tool is set for appropriate operation in the Kanto District, Japan (50 Hz), and that the electrically powered tool is used in the Kansai District (60 Hz), and then, the operating lever must be moved a longer distance at the outset in the Kansai District. Also, the running speed rises rapidly from the outset to the maximum speed. This causes a significant inconvenience in controlling the electrically powered tool.